No. 4 Electronic Switching System
The No. 4 Electronic Switching System is a class 4 telephone electronic switching system, the first digital electronic toll switch introduced by Western Electric for long-distance switching. It was introduced in Chicago in 1976, to replace the 4a crossbar switch; the last of 145 in the AT&T network was installed in 1999 in Atlanta. Half of the switches were manufactured in Lisle and the other half in Oklahoma City, Oklahoma. At the time of the Bell System divestiture, most of the 4ESS switches became assets of AT&T as part of the long-distance network, while others remained in the RBOC networks. 150 4ESS switches remained in service in the United States in 2007. The 4ESS has three major components: the processor, the file store, known as the attached processor system, the peripheral units; the processor acts as the CPU for the switch. The processor includes a central control, call stores, program stores. In addition it had access to additional units through the auxiliary unit bus and peripheral unit bus.
A master control console provides office technicians access to the switch through the processor peripheral interface. Early versions used the same 1A processor. All existing switches have been subsequently upgraded to use the 1B processor; the file store provides long term storage of the processor programs and office data. It was replaced by the 4E attached processor system; the 4EAPS is a 3B computer running 4EAPS application software on the DMERT operating system. The 4EAPS interfaces to the 4ESS processor via the attached processor interface units; the "1A file store" became partitions on the 3B computer disks. At first the 4EAPS just provided "file store" but soon it provided access to the common-network interface ring to provide common-channel signaling; the 4EAPS used the 3B20D computer. These were all converted to the 3B21D around 1995; the peripheral units include units that interface to the central control over the peripheral unit bus. This includes the common channel interface signaling terminal, signal processors, time-slot interchanges and time multiplexed switches.
It includes equipment not directly on the PUB including terminating equipment used to connect the switch to the transport network and the TSIs and TMSs, which perform the "time-space-time" switching function. Timing is provided by high accuracy network clock. 4ESS development began in about 1970 in Naperville, Illinois under the direction of Henry Earle Vaughan. AT&T Long Distance was the primary customer for the switch. Driving development from the customer's perspective was AT&T VP Billy Oliver. Previous tandem switching systems the No. 4 Crossbar switch, used analog voice signaling. The decision to switch in a digital voice format was controversial at the time, both from a technical and economic viewpoint. Visionaries such as Vaughn and Oliver recognized that the network would become digital, requiring digital switching technologies; the last 4ESS was installed in suburban Atlanta, GA in 1999 as a toll tandem for AT&T. At the peak of the product's life time in 1999, AT&T employed 145 4ESS switches in its long-haul network, several were owned by various Regional Bell Operating Companies.
AT&T replaced or supplemented the 4ESS toll tandem switches with 5ESS switches, which featured an advanced design, are used as edge switches in the network. Most RBOCs who used 4ESS tandems have replaced them with Class 5 systems of other manufacturers, e.g. Nortel; as of 2014, AT&T operates and maintains one hundred 4ESS switches in the public switched telephone network. The Nokia N4E Solution is the ATCA based next-generation 4ESS for AT&T; the N4E includes the 4E APS and 4ESS software but replaces the 1B processor and the peripheral units which run in emulated environments on an ATCA blade. The N4E is based on the Alcatel-Lucent gateway platform (7520 Media Gateway, 1310 Operations and Management Console – Plus and the 5400 Linux Control Platform and includes other elements such as MRV console terminal servers. 1ESS switch 5ESS switch Telephone exchange Bell System Practices Special issue on the 4ESS switch BSTJ, Sept. 1977, Vol. 56, No. 7 Bell System commercial about the 4ESS switch on YouTube http://www.phworld.org/switch/weess.htm ATT Crash
Illinois is a state in the Midwestern and Great Lakes region of the United States. It has the fifth largest gross domestic product, the sixth largest population, the 25th largest land area of all U. S. states. Illinois is noted as a microcosm of the entire United States. With Chicago in northeastern Illinois, small industrial cities and immense agricultural productivity in the north and center of the state, natural resources such as coal and petroleum in the south, Illinois has a diverse economic base, is a major transportation hub. Chicagoland, Chicago's metropolitan area, encompasses over 65% of the state's population; the Port of Chicago connects the state to international ports via two main routes: from the Great Lakes, via the Saint Lawrence Seaway, to the Atlantic Ocean and from the Great Lakes to the Mississippi River, via the Illinois Waterway to the Illinois River. The Mississippi River, the Ohio River, the Wabash River form parts of the boundaries of Illinois. For decades, Chicago's O'Hare International Airport has been ranked as one of the world's busiest airports.
Illinois has long had a reputation as a bellwether both in social and cultural terms and, through the 1980s, in politics. The capital of Illinois is Springfield, located in the central part of the state. Although today's Illinois' largest population center is in its northeast, the state's European population grew first in the west as the French settled the vast Mississippi of the Illinois Country of New France. Following the American Revolutionary War, American settlers began arriving from Kentucky in the 1780s via the Ohio River, the population grew from south to north. In 1818, Illinois achieved statehood. Following increased commercial activity in the Great Lakes after the construction of the Erie Canal, Chicago was founded in the 1830s on the banks of the Chicago River at one of the few natural harbors on the southern section of Lake Michigan. John Deere's invention of the self-scouring steel plow turned Illinois's rich prairie into some of the world's most productive and valuable farmland, attracting immigrant farmers from Germany and Sweden.
The Illinois and Michigan Canal made transportation between the Great Lakes and the Mississippi River valley faster and cheaper, new railroads carried immigrants to new homes in the country's west and shipped commodity crops to the nation's east. The state became a transportation hub for the nation. By 1900, the growth of industrial jobs in the northern cities and coal mining in the central and southern areas attracted immigrants from Eastern and Southern Europe. Illinois was an important manufacturing center during both world wars; the Great Migration from the South established a large community of African Americans in the state, including Chicago, who founded the city's famous jazz and blues cultures. Chicago, the center of the Chicago Metropolitan Area, is now recognized as a global alpha-level city. Three U. S. presidents have been elected while living in Illinois: Abraham Lincoln, Ulysses S. Grant, Barack Obama. Additionally, Ronald Reagan, whose political career was based in California, was born and raised in the state.
Today, Illinois honors Lincoln with its official state slogan Land of Lincoln, displayed on its license plates since 1954. The state is the site of the Abraham Lincoln Presidential Library and Museum in Springfield and the future home of the Barack Obama Presidential Center in Chicago. "Illinois" is the modern spelling for the early French Catholic missionaries and explorers' name for the Illinois Native Americans, a name, spelled in many different ways in the early records. American scholars thought the name "Illinois" meant "man" or "men" in the Miami-Illinois language, with the original iliniwek transformed via French into Illinois; this etymology is not supported by the Illinois language, as the word for "man" is ireniwa, plural of "man" is ireniwaki. The name Illiniwek has been said to mean "tribe of superior men", a false etymology; the name "Illinois" derives from the Miami-Illinois verb irenwe·wa - "he speaks the regular way". This was taken into the Ojibwe language in the Ottawa dialect, modified into ilinwe·.
The French borrowed these forms, changing the /we/ ending to spell it as -ois, a transliteration for its pronunciation in French of that time. The current spelling form, began to appear in the early 1670s, when French colonists had settled in the western area; the Illinois's name for themselves, as attested in all three of the French missionary-period dictionaries of Illinois, was Inoka, of unknown meaning and unrelated to the other terms. American Indians of successive cultures lived along the waterways of the Illinois area for thousands of years before the arrival of Europeans; the Koster Site demonstrates 7,000 years of continuous habitation. Cahokia, the largest regional chiefdom and urban center of the Pre-Columbian Mississippian culture, was located near present-day Collinsville, Illinois, they built an urban complex of more than 100 platform and burial mounds, a 50-acre plaza larger than 35 football fields, a woodhenge of sacred cedar, all in a planned design expressing the culture's cosmology.
Monks Mound, the center of the site, is the largest Pre-Columbian structure north of the Valley of Mexico. It is 100 feet high, 951 feet long, 836 feet wide, covers 13.8 acres. It contains about 814,000 cubic yards of earth, it was topped by a structure thought to have measured about 105 feet in length and 48 feet in width, covered an area 5,000 square feet, been as much as 50 feet high, making its peak 150 feet above the level of the pl
A reed relay is a type of relay that uses an electromagnet to control one or more reed switches. The contacts are of magnetic material and the electromagnet acts directly on them without requiring an armature to move them. Sealed in a long, narrow glass tube, the contacts are protected from corrosion; the glass envelope may contain multiple reed switches or multiple reed switches can be inserted into a single bobbin and actuate simultaneously. Reed switches have been manufactured since the 1930s. Compared with armature-based relays, reed relays can switch much faster, as the moving parts are small and lightweight, although switch bounce is still present, they require less operating power and have lower contact capacitance. Their current handling capacity is limited but, with appropriate contact materials, they are suitable for "dry" switching applications, they are mechanically simple, making for long life. A few million reed relays were used from the 1930s to the 1960s for memory functions in Bell System electromechanical telephone exchanges.
A multiple-reed relay was used, with one of the reeds latching the relay, the other or others performing logic or memory functions. Most reed relays in the crossbar switching systems of the 1940s through the 1970s were packaged in groups of five; such a "reed pack" was able to store one decimal digit, encoded in a two-out-of-five code for easy validity checking by wire spring relay logic. Such an electrically latching reed relay requires continuous power to maintain state, unlike magnetically latching relays, such as ferreed or the remreed. In the Bell System Stored Program Control exchange systems of the 1970s, reed relays were no longer needed for data storage, but tens of millions of them were packaged in arrays for voice path switching. In the 1ESS switch, the cores were made of a magnetically remanent alloy, so the relay could latch magnetically instead of latching electrically; this "Ferreed" method allowed both contacts to be used for voice path. The coils were wired for coincident current selection similar to a magnetic core memory, so operating the contacts for one crosspoint would release the other crosspoints in its row and column.
Each input of the array had, besides the two talk wires, a P lead for controlling the crosspoints on that level. Two coils on each crosspoint were wired in series to the P lead; each output of the array had a P lead with two coils on each crosspoint of that output level. The two windings controlled by the same level were unequal, were wound around opposite ends of the reed, in opposing polarity; when a pulse passed through the crosspoints of a level, the two ends of each reed were magnetized north to north or south to south, thus repelled each other and opened the crosspoint in all except the selected crosspoint. The selected crosspoint had current passing through both its input P lead and its output P lead, thus through all four windings. On each end of the ferreed, the windings provided by the two different P leads were opposed to each other, the greater one predominated when both were energized; this being the input P lead at one end of the ferreed, the output P lead at the other end, the two ends of that particular ferreed were magnetized north to south, hence attracted each other and closed the contact.
Current was applied by the pulser only to set up the connection. The P leads remained dry and the crosspoint remained closed until such time as another connection was made involving one of the levels; because the individual crosspoints were more expensive than those of crossbar switches, while the control circuitry was cheaper, reed arrays had fewer crosspoints and were more numerous. This required them to be arranged in more stages. Thus, while a telephone call in a typical crossbar exchange like 5XB passed through four switches, a call in a reed system such as 1ESS passed through eight. In the 1AESS, the reeds were of remanent magnetic material; this "Remreed" design allowed further reduction in power consumption. A "grid" of 1024 2-wire crosspoints, arranged as two stages of eight 8x8 switches, was permanently packaged in a box. Despite the sealed contacts, plating with silver rather than with precious metals resulted in reed arrays being less reliable than crossbar switches; when one crosspoint failed, the grid box was replaced as a unit, either repaired at a local workbench or shipped to a repair shop.
Stromberg-Carlson made the similar ESC system. Reed relays were extensively used in the British TXE family of telephone exchanges. Reed arrays passed out of use in the mid-1990s, being unnecessary in digital telephone systems such as DMS-100 and 5ESS switch. Reed relays have continued in their uses outside the telephone industry, such as for automatic test equipment and electronic instrumentation due to their hermetic seal, fast operate time, extended life to 109 operations and consistent contact performance. Reed relays have found numerous applications in RF and microwave switching applications, they are used in applications which make use of their low leakage current such as photomultiplier detectors and other low current handling circuits. Reed switches can be manufactured to withstand several kilovolts and are still used as high-voltage relays in place of more costly sulfur hexafluoride or vacuum relays. Mercury-wetted reed relay PRX Reed receiver TXE Vibrator Glossary of Reed Switch and Related Terms Advanced information about Reed Switches Patent for remanent reed relay design Bell Syste
A semiconductor device is an electronic component that exploits the electronic properties of semiconductor material, principally silicon and gallium arsenide, as well as organic semiconductors. Semiconductor devices have replaced vacuum tubes in most applications, they use electrical conduction in the solid state rather that the gaseous state or thermionic emission in a vacuum. Semiconductor devices are manufactured both as single discrete devices and as integrated circuits, which consist of two or more devices—which can number in the billions—manufactured and interconnected on a single semiconductor wafer. Semiconductor materials are useful because their behavior can be manipulated by the deliberate addition of impurities, known as doping. Semiconductor conductivity can be controlled by the introduction of an electric or magnetic field, by exposure to light or heat, or by the mechanical deformation of a doped monocrystalline silicon grid. Current conduction in a semiconductor occurs due to mobile or "free" electrons and electron holes, collectively known as charge carriers.
Doping a semiconductor with a small proportion of an atomic impurity, such as phosphorus or boron increases the number of free electrons or holes within the semiconductor. When a doped semiconductor contains excess holes, it is called a p-type semiconductor. A majority of mobile charge carriers have negative charge; the manufacture of semiconductors controls the location and concentration of p- and n-type dopants. The connection of n-type and p-type semiconductors form a p–n junctions. Semiconductor devices made per year have been growing by 9.1% on average since 1978, shipments in 2018 are predicted for the first time to exceed 1 trillion, meaning that well over 7 trillion has been made to date, in just in the decade prior. A semiconductor diode is a device made from a single p–n junction. At the junction of a p-type and an n-type semiconductor there forms a depletion region where current conduction is inhibited by the lack of mobile charge carriers; when the device is forward biased, this depletion region is diminished, allowing for significant conduction, while only small current can be achieved when the diode is reverse biased and thus the depletion region expanded.
Exposing a semiconductor to light can generate electron–hole pairs, which increases the number of free carriers and thereby the conductivity. Diodes optimized to take advantage of this phenomenon are known as photodiodes. Compound semiconductor diodes can be used to generate light, as in light-emitting diodes and laser diodes. Bipolar junction transistors are formed from two p–n junctions, in either n–p–n or p–n–p configuration; the middle, or base, region between the junctions is very narrow. The other regions, their associated terminals, are known as the emitter and the collector. A small current injected through the junction between the base and the emitter changes the properties of the base-collector junction so that it can conduct current though it is reverse biased; this creates a much larger current between the collector and emitter, controlled by the base-emitter current. Another type of transistor, the field-effect transistor, operates on the principle that semiconductor conductivity can be increased or decreased by the presence of an electric field.
An electric field can increase the number of free electrons and holes in a semiconductor, thereby changing its conductivity. The field may be applied by a reverse-biased p–n junction, forming a junction field-effect transistor or by an electrode insulated from the bulk material by an oxide layer, forming a metal–oxide–semiconductor field-effect transistor; the MOSFET, a solid-state device, is the most used semiconductor device today. The gate electrode is charged to produce an electric field that controls the conductivity of a "channel" between two terminals, called the source and drain. Depending on the type of carrier in the channel, the device may be an n-channel or a p-channel MOSFET. Although the MOSFET is named in part for its "metal" gate, in modern devices polysilicon is used instead. By far, silicon is the most used material in semiconductor devices, its combination of low raw material cost simple processing, a useful temperature range makes it the best compromise among the various competing materials.
Silicon used in semiconductor device manufacturing is fabricated into boules that are large enough in diameter to allow the production of 300 mm wafers. Germanium was a used early semiconductor material but its thermal sensitivity makes it less useful than silicon. Today, germanium is alloyed with silicon for use in very-high-speed SiGe devices. Gallium arsenide is widely used in high-speed devices but so far, it has been difficult to form large-diameter boules of this material, limiting the wafer diameter to sizes smaller than silicon wafers thus making mass production of GaAs devices more expensive than silicon. Other less common materials are in use or under investigation. Silicon carbide has found some application as the raw material for blue light-emitting diodes and is being investigated for use in semiconductor devices that could withstand high operating temperatures and environments with the presence of significant levels of ionizing radiation. IMPATT diodes have been fabr
General Services Administration
The General Services Administration, an independent agency of the United States government, was established in 1949 to help manage and support the basic functioning of federal agencies. GSA supplies products and communications for U. S. government offices, provides transportation and office space to federal employees, develops government-wide cost-minimizing policies and other management tasks. GSA employs about 12,000 federal workers and has an annual operating budget of $20.9 billion. GSA oversees $66 billion of procurement annually, it contributes to the management of about $500 billion in U. S. federal property, divided chiefly among 8,700 owned and leased buildings and a 215,000 vehicle motor pool. Among the real estate assets managed by GSA are the Ronald Reagan Building and International Trade Center in Washington, D. C. – the largest U. S. federal building after the Pentagon – and the Hart-Dole-Inouye Federal Center. GSA's business lines include the Federal Acquisition Service and the Public Buildings Service, as well as several Staff Offices including the Office of Government-wide Policy, the Office of Small Business Utilization, the Office of Mission Assurance.
As part of FAS, GSA's Technology Transformation Services helps federal agencies improve delivery of information and services to the public. Key initiatives include FedRAMP, Cloud.gov, the USAGov platform, Data.gov, Performance.gov, Challenge.gov. GSA is a member of the Procurement G6, an informal group leading the use of framework agreements and e-procurement instruments in public procurement. In 1947 President Harry Truman asked former President Herbert Hoover to lead what became known as the Hoover Commission to make recommendations to reorganize the operations of the federal government. One of the recommendations of the commission was the establishment of an "Office of the General Services." This proposed office would combine the responsibilities of the following organizations: U. S. Treasury Department's Bureau of Federal Supply U. S. Treasury Department's Office of Contract Settlement National Archives Establishment All functions of the Federal Works Agency, including the Public Buildings Administration and the Public Roads Administration War Assets AdministrationGSA became an independent agency on July 1, 1949, after the passage of the Federal Property and Administrative Services Act.
General Jess Larson, Administrator of the War Assets Administration, was named GSA's first Administrator. The first job awaiting Administrator Larson and the newly formed GSA was a complete renovation of the White House; the structure had fallen into such a state of disrepair by 1949 that one inspector of the time said the historic structure was standing "purely from habit." Larson explained the nature of the total renovation in depth by saying, "In order to make the White House structurally sound, it was necessary to dismantle, I mean dismantle, everything from the White House except the four walls, which were constructed of stone. Everything, except the four walls without a roof, was stripped down, that's where the work started." GSA worked with President Truman and First Lady Bess Truman to ensure that the new agency's first major project would be a success. GSA completed the renovation in 1952. In 1986 GSA headquarters, U. S. General Services Administration Building, located at Eighteenth and F Streets, NW, was listed on the National Register of Historic Places, at the time serving as Interior Department offices.
In 1960 GSA created the Federal Telecommunications System, a government-wide intercity telephone system. In 1962 the Ad Hoc Committee on Federal Office Space created a new building program to address obsolete office buildings in Washington, D. C. resulting in the construction of many of the offices that now line Independence Avenue. In 1970 the Nixon administration created the Consumer Product Information Coordinating Center, now part of USAGov. In 1974 the Federal Buildings Fund was initiated, allowing GSA to issue rent bills to federal agencies. In 1972 GSA established the Automated Data and Telecommunications Service, which became the Office of Information Resources Management. In 1973 GSA created the Office of Federal Management Policy. GSA's Office of Acquisition Policy centralized procurement policy in 1978. GSA was responsible for emergency preparedness and stockpiling strategic materials to be used in wartime until these functions were transferred to the newly-created Federal Emergency Management Agency in 1979.
In 1984 GSA introduced the federal government to the use of charge cards, known as the GMA SmartPay system. The National Archives and Records Administration was spun off into an independent agency in 1985; the same year, GSA began to provide governmentwide policy oversight and guidance for federal real property management as a result of an Executive Order signed by President Ronald Reagan. In 2003 the Federal Protective Service was moved to the Department of Homeland Security. In 2005 GSA reorganized to merge the Federal Supply Service and Federal Technology Service business lines into the Federal Acquisition Service. On April 3, 2009, President Barack Obama nominated Martha N. Johnson to serve as GSA Administrator. After a nine-month delay, the United States Senate confirmed her nomination on February 4, 2010. On April 2, 2012, Johnson resigned in the wake of a management-deficiency report that detailed improper payments for a 2010 "Western Regions" training conference put on by the Public Buildings Service in Las Vegas.
In July 1991 GSA contractors began the excavation of what is now the Ted Weiss Federal Building in New York City. The planning for that buildin
A telephone exchange is a telecommunications system used in the public switched telephone network or in large enterprises. An exchange consists of electronic components and in older systems human operators that interconnect telephone subscriber lines or virtual circuits of digital systems to establish telephone calls between subscribers. In historical perspective, telecommunication terms have been used with different semantics over time; the term telephone exchange is used synonymously with central office, a Bell System term. A central office is defined as a building used to house the inside plant equipment of several telephone exchanges, each serving a certain geographical area; such an area has been referred to as the exchange. Central office locations may be identified in North America as wire centers, designating a facility from which a telephone obtains dial tone. For business and billing purposes, telephony carriers define rate centers, which in larger cities may be clusters of central offices, to define specified geographical locations for determining distance measurements.
In the United States and Canada, the Bell System established in the 1940s a uniform system of identifying central offices with a three-digit central office code, used as a prefix to subscriber telephone numbers. All central offices within a larger region aggregated by state, were assigned a common numbering plan area code. With the development of international and transoceanic telephone trunks driven by direct customer dialing, similar efforts of systematic organization of the telephone networks occurred in many countries in the mid-20th century. For corporate or enterprise use, a private telephone exchange is referred to as a private branch exchange, when it has connections to the public switched telephone network. A PBX is installed in enterprise facilities collocated with large office spaces or within an organizational campus to serve the local private telephone system and any private leased line circuits. Smaller installations might deploy a PBX or key telephone system in the office of a receptionist.
In the era of the electrical telegraph, post offices, railway stations, the more important governmental centers, stock exchanges few nationally distributed newspapers, the largest internationally important corporations and wealthy individuals were the principal users of such telegraphs. Despite the fact that telephone devices existed before the invention of the telephone exchange, their success and economical operation would have been impossible on the same schema and structure of the contemporary telegraph, as prior to the invention of the telephone exchange switchboard, early telephones were hardwired to and communicated with only a single other telephone. A telephone exchange is a telephone system located at service centers responsible for a small geographic area that provided the switching or interconnection of two or more individual subscriber lines for calls made between them, rather than requiring direct lines between subscriber stations; this made it possible for subscribers to call each other at businesses, or public spaces.
These made telephony an available and comfortable communication tool for everyday use, it gave the impetus for the creation of a whole new industrial sector. As with the invention of the telephone itself, the honor of "first telephone exchange" has several claimants. One of the first to propose a telephone exchange was Hungarian Tivadar Puskás in 1877 while he was working for Thomas Edison; the first experimental telephone exchange was based on the ideas of Puskás, it was built by the Bell Telephone Company in Boston in 1877. The world's first state-administered telephone exchange opened on November 12, 1877 in Friedrichsberg close to Berlin under the direction of Heinrich von Stephan. George W. Coy designed and built the first commercial US telephone exchange which opened in New Haven, Connecticut in January, 1878; the switchboard was built from "carriage bolts, handles from teapot lids and bustle wire" and could handle two simultaneous conversations. Charles Glidden is credited with establishing an exchange in Lowell, MA. with 50 subscribers in 1878.
In Europe other early telephone exchanges were based in London and Manchester, both of which opened under Bell patents in 1879. Belgium had its first International Bell exchange a year later. In 1887 Puskás introduced the multiplex switchboard.. Exchanges consisted of one to several hundred plug boards staffed by switchboard operators; each operator sat in front of a vertical panel containing banks of ¼-inch tip-ring-sleeve jacks, each of, the local termination of a subscriber's telephone line. In front of the jack panel lay a horizontal panel containing two rows of patch cords, each pair connected to a cord circuit; when a calling party lifted the receiver, the local loop current lit a signal lamp near the jack. The operator responded by inserting the rear cord into the subscriber's jack and switched her headset into the circuit to ask, "Number, please?" For a local call, the operator inserted the front cord of the pair into the called party's local jack and started the ringing cycle. For a long distance call, she plugged into a trunk circuit to connect to another operator in another bank of boards or at a remote central office.
In 1918, the average time to complete the connection for a long-distance call was 15 minutes. Early manual switchboards required the operator to operate listening keys and ringing keys, but by the late 1910s and 1920s, advances in switchboard technology led to features which allowed the call to be automatic
The Panel Machine Switching System is an early type of automatic telephone exchange for urban service, introduced in the Bell System in the 1920s. It was developed by Western Electric Laboratories, the forerunner of Bell Labs, in the U. S. in parallel with the Rotary system at International Western Electric in Belgium before World War I. Both systems had many features in common; the Panel switch was named for its tall panels. Between each strip was placed an insulating layer, which kept each metal strip electrically isolated from the ones above and below; these terminals were arranged in banks. Each bank contained 100 sets of terminals, for a total of 500 sets of terminals per frame. At the bottom, the frame had two electric motors to drive sixty selectors up and down by electromagnetically controlled clutches; as calls were completed through the system, selectors moved vertically over the sets of terminals until they reached the desired location, at which point the selector stopped its upward travel, selections progressed to the next frame, until the called subscriber's line was reached.
The first Panel-type exchanges were placed in service in Newark, New Jersey, on January 16, 1915 at the Mulberry central office, on June 12 in the Waverly central office. These systems were semi-mechanical. Operators answered calls and keyed the station number into the panel switch, which completed the call; the first machine-switching Panel systems using common control principles were placed in service in Omaha, Nebraska in December 1921, followed by the PEnnsylvania exchange in New York City in October 1922. Most Panel installations were replaced by modern systems during the 1970s; the last Panel switch, located in the Bigelow central office in Newark, was decommissioned by 1983. When a subscriber removed his or her handset from hook-switch, a circuit was closed from the central office, through the subscriber's telephone, back to the central office; the closing of this circuit operated a line relay. This caused a selector on the line finder frame to begin hunting for the terminal on which the subscriber was located.
A sender was selected, which provided dial tone for the caller once their line was found. Having located and connected to the proper terminal for the subscriber's line, the line finder operated a "cutoff" relay, which prevented that telephone from being called, should another subscriber happen to dial their number while they were off-hook. After dial tone was heard, the subscriber could begin dialing. Depending on the local numbering system, the sender required either six or seven digits in order to complete the call; as the subscriber dialed, relays in the sender counted and stored the digits for usage. As soon as the two, or three digits of the office code were dialed and stored, the sender performed a lookup against a translator or decoder; the translator or decoder took the office code as input, returned data to the sender that contained the parameters for connecting to the called central office. This data was stored in the translator or decoder by a method of cross-connects on terminal strips.
After the sender received the data provided by the translator, the sender used this information to guide the district selector and office selector to the location of the terminals that would connect the caller to the central office where the terminating line was located. The sender stored and utilized other information pertaining to the electrical requirements for signaling over the newly established connection, the rate at which the subscriber should be billed, should the call complete. On the district or office selectors themselves, idle outgoing trunks were picked by the "sleeve test" method. After being directed by the sender to the correct group of terminals corresponding to the outgoing trunks to the called office, the selector continued moving upward through a number of terminals, checking for one with an un-grounded sleeve lead selecting and grounding it. If all the trunks were busy, the selector hunted to the end of the group, sent back an "all circuits busy" tone. There was no provision for alternate routing as in earlier manual systems and more sophisticated mechanical ones.
Once the connection to the terminating office was established, the sender used the last four digits of the telephone number to reach the called party. It did so by converting the digits into specific locations on the remaining incoming and final frames. After the connection was established all the way to the final frame, the called party's line was tested for busy. If the line was not busy, the incoming selector circuit sent ringing voltage forward to the called party's line and waited for the called party to answer their telephone. If the called party answered, supervision signals were sent backwards through the sender, to the district frame, which established a talking path between both subscribers, charged the calling party for the call. At this time, the sender was released, could be used again in service of an new call. If the called subscriber's line was busy, the final selector sent a busy signal back to the called party to alert them that the caller was on the phone and could not accept their call.
As in the Strowger system, each central office could address up to 10,000 numbered lines, requiring four digits for each subscriber station. The panel system was designed to connect calls in a local metropolitan calling area; each office was assigned a two- or three-digit office code, which indicated to the system the central office in which the desired party was located. Callers dialed the